The present invention relates to a method and apparatus for reducing corrosion in a tank containing water. The present invention is particularly applicable to water cooling towers.
There are many manufacturing plants, power generating systems and air conditioning installations which require efficient cooling. This cooling is generally achieved by using recirculating water systems to obtain the most efficient utilisation of water.
The typical system consists of a basin or sump from which a pump circulates water through pipes to other components, which are normally one or more heat exchangers and a cooling tower, in which the warmed water is sprayed into an air stream. Cooling is achieved by the evaporation of the circulating water. The cooled water falls into the basin to complete the circuit. Any make up water needed, to replace water evaporated or bled from the system, is added into the basin. As water evaporates from the system the concentration of natural salts is increased.
Scale formation and/or silt accumulation reduces the efficiency and performance of the cooling system due to restricted heat transfer and interference with water flow.
The evaporation of water from the cooling tower increases the concentration of natural salts in the circulating water. As this process continues, the natural calcium alkaline hardness in the water will pass its solubility limit and scale formation will take place. The higher temperatures at the heat transfer surfaces increases the rate of scale formation on these surfaces, thereby reducing the efficiency and operating capacity of the unit.
Often scale formation is accompanied by the trapping of airborne debris, removed from the cooling air by the scrubbing action of the water which is sprayed through the tower. This inclusion of corrosion product debris or a combination of these materials is often held more tightly together by organic growths.
The accumulation of debris in the absence of scaling results in silt creation.
Generally it is necessary to bleed off some of the concentrated water to limit scale formation and silting problems. In many cases the volumes of water which have to be bled off to prevent scaling are excessive and very precise control is required to achieve satisfactory results. This generally results in the method being impractical.
An extension of this method is the addition of acids to the circulating water. Very precise addition, mixing and control is needed to prevent severe damage to the system when acids are added. There are also problems of safety arising from the need to store, handle and apply these acids.
Target concentration for the addition of acid or bleed off of water may be calculated using the Langelier Index, however, this technique is far from satisfactory as the allowable margins of error are extremely limited. These margins are greatly exceeded if there are any variations in the water composition. These methods also impose severe control requirements, especially in high output, smaller water capacity systems which have come into use in recent times, and which make such control extremely difficult.
Most waters are corrosive and this problem may be aggravated by absorption of acidic gases and other impurities from the cooling air. In addition, many of the salts in solution in the water enhance corrosion, particularly chlorides. Corrosive attack on the metal surfaces reduces the life of the plant necessitating repairs or complete replacement of the plant.
Corrosion product build up also interferes with heat transfer and water distribution, and this may be amplified by the inclusion of scale, silt, airborn material or organic matter.
The control of corrosion by achieving desirable calcium bicarbonate concentrations in circulating water using the Langelier Index is an excellent method in theory, however, in practice it has been found to be very difficult to apply given, the very precise control requirements needed to give the desired protection without scale formation. Attempts to widen the control parameters using scale modifiers have only been partially successful.
The use of various corrosion inhibitors such as chromates, zinc etc. has met with some success, however, increasingly tighter controls on the discharge of toxic wastes has led to severe limitations on the use of these chemicals. In addition, the emission of water droplets or aerosols from the air exhausts of water cooling towers containing toxic chemicals have caused concern to health authorities, and pollution control regulations are now increasingly preventing the use of these inhibitors.
The presence of organic contamination of the water in the recirculating water system provides the basis for very rapid growth of various forms of biological contaminants, such as slime, algae and bacteria. The conditions of temperature and aeration present in these systems also helps to promote the growth of biological contaminants, as does exposure to sunlight.
Algae and slimes themselves reduce heat transfer. They also increase the mass of scale and silt and bind it more firmly to internal surfaces of the cooling system further aggravating heat transfer problems. Corrosion is frequently accelerated by the presence of organic matter, which can also interfere with water distribution through the cooling tower, reducing the cooling effect. Bacterial contamination of the water circulating systems is also of great concern. Many bacteria are highly dangerous and in recent years special attention has been drawn to the pathogenicity of some strains of Legionnella, particularly for the elderly and others whose resistance to disease is diminished. It is now generally felt that many deaths caused by Legionnella had been previously attributed to other causes.
Bacteria pass into the exhaust air from a tower as aerosols. These can be sucked in by fresh air intakes of air conditioning systems or can drift into the street and can then be inhaled by people possibly causing infection. Bacteria may also escape from the system in the bleed off water and in aerosols created when the system is washed out or maintenance is taking place.
A wide range of chemicals such as chlorine, have been used to control biological growth with reasonable success in relation to algae and slime but with varying effect in relation to bacterial contamination.
Due to greater public awareness of the dangers of microbial contamination of cooling systems water in air conditioning systems, attention has been focussed on other methods of disinfecting the water present in these systems. One method of disinfection involves of the use of ozone.
Apart from being a very effective biocide ozone provides a number of other benefits. Ozone has been found to passivate metal surfaces with an oxidised film or layer, and has been shown to have scale inhibiting properties. Algae, slime and other impurities in the water additionally act as a glue for solids such as calcium and other particulate matter to form layers of scale. By oxidising all slime and algae, ozone prevents such solids from being glued into scale layers on the walls of cooling towers and condensed water piping. The same properties also de-scale old cooling water systems.
Another benefit achieved in the use of ozone is that ozone has a microfloculating effect on the cooling water which causes particulate matter to coagulate and settle to the bottom of the cooling tower or strainer as a fluidized powder or slurry.
It is known that the presence of calcium bicarbonate will inhibit corrosion of metal surfaces. However, the use of an excess of calcium bicarbonate typically leads to the formation of scale.